Telephone instrument circuit



June 6, 1944- J M N 2,350,604

TELEPHONE INSTRUMENT CIRCUIT '7 Filed siSt. 3,1942

INVENTOR EMANUEL FRYDMAN .i ZZ

ATTORNEY Patented June 6, 1944 TELEPHONE INSTRUMENT CIRCUIT EmanuelFrydman, Liverpool, England, assignor, by mesne assignments, toAutomatic Electric Laboratories, Inc., Chicago, 'IlL, a'corporation ofDelaware 1 Application September 3, 1942, Serial'No. 457,119 In GreatBritain September 6, 1941 5 Claims. ('Cl..179-81) The present inventionrelates to telephone substation instrument circuits suitable for use insystems which follow the usual practice of *employing low frequencyalternating, current for signalling a Wanted party by the ringing of abell or the like.

The invention finds particular application to two-party line circuitsfor use in automatic telephone systems in which the general arrangementis that the bells of the two parties sharing a line are earthed, onebell being'connecte'd to one leg of the line and the other bell to the"other leg. In order to avoidthe'completion' of direct current earthbiassing circuits by way of the telephone bells, it is then desirable toconnect a condenser in the earth circuit of each of the bells. Withthese arrangements, however, the changes in distributed exchange batteryvoltage at the substation end of a party line due to one of the partiessetting up a call by dialling cause tinkling of the bells both to thecalling party and of the other party sharing the line and it is possiblefor the tinkling of the second bell to be mistaken for ringing by thesecond party.

In telephone instrument circuits involving a single subscriber per line,the signalling bell is usually connected in series with a condenseracross the line and arrangements are made to provide a low resistanceshunt path for the bell during dialling, so as to preventtinkling duringthis time. In this case, however, it is possible for bell tinkling to beproduced by branches of trees making intermittent contact with open'wirelines and also by maintenance "men testing or working on the line. Theinvention has for its main object the provision of simple and efficientmeans for preventing the annoyance of bell tinkling'due-to the abovementioned and similar conditions.

According to the invention in a telephone system employing low frequencyalternating current for signalling purposes, the circuit for-= the bellor the like in a substation instrument circuit includes a non-linearresistance element arranged to present a high resistance to the exchangebattery voltage but to break down at the signalling current voltage.

The invention will be better understood from the following descriptionof one method of carrying it into efiect, reference being had to theaccompanying drawing which shows circuits of the substation end of atwo-party line assumedto extend to an automatic telephone exchange; The

circuits of one of the party line stations =A- i s shown in full, whileas regards'the other station B, only the bell circuit of this ha beenshown, since the equipment thereat is identical With that of the firststation, the only difieren'ce being that the line conductors arereversed'so 7 that the bell circuit connects with the'opposite leg ofthe line. Considering now the equipment at the first party linestation-A, this comprises the usual transmitter T, receiver R,three-winding antia side tone induction coil comprising windings ICI,

I02 and I03, resistors YA and YB and switchhook contacts SHI and SHZwhich close when the receiver or handset, as the case may-beyis lifted.The subscribers dia'l impulse contacts are designated IMF and othercontacts ofthedi'al (notshown) are closed when the dial is cirnor maland serve in well-known manner to shore circuit the transmitter T andreceiverR during dialling. As regards the bell ringing circuitjthiscomprises the usual condenser l0 and bell H,

but'in accordance with the invention "the addi tional condenser throughwhich the bell is normally connected to earth is now replaced byanon-linear resistance element l2. Similar-"Ic mark apply in connectionwith the second party line station B with the difierence'that'the bell2| which is earthed via non-linear resistance ele ment 22 is connectedvia condenser 20 to the 'opposite leg of the line compared with thebell' l I.

In accordance with usual automatic exchange practice, alternatingcurrent at a voltage -'offth! order of volts is assumed to be used forring ing' purposes, while as will be better appreciated later in thedescription, the voltages which ordi narily produce bell tinkling cannot-exceed 'the voltage of the exchange batterywhichisnormally in theneighbourhood of say 50 volts. 'The function of the non-linearresistance de vices l2 and 22 is essentially to discriminate 'betweenringing and tinkling voltages so that on the one hand a low resistancewill be offered: to ringing currents and on the other hand 'avery highresistance will be presented to currents 22 take the form of small solidunits "ofnonlinear resistance material comprising amixture" of siliconcarbide with carbon and/or tungsten or molybdenum'which is agglomeratedunder pressure and subsequently baked. Such units ing difliculties whichare encountered when the instrument bells are connected to earth viacon-i:

densers.

e ing now the telephone instrument cir: cult of party line subscriber A,the condenser I will normally be charged up to. the exchange bat teryvoltage of 50 volts in series with the line relay associated with theparty line in question,

but the bell does not tinkle'when this circuit is first closed owing tothe slow rate of charging of condenser I0 due to the high resistance ofunit I2. telephone instrument is looped across the line via theswitchhook contacts SH2 and condenser IQ, is discharged over contactsSHI, resistance YA, induction coil'winding 1C2, resistance YB,

I transmitter T, and dial impulsing contacts IMP. 5 At the main exchangethe loop thus completed operates the line relay and causes a firstselector to be seized in well-known ,manner, whereupon battery isconnected to the negative leg of the linevia one windin'gof the selectorimpulsing relay and earth is connected to the positive leg of the linevia' the other winding of the relay.

When subscriber A dials the first digit of the When the subscriberinitiates a call, the" itive line to earth at the main exchange, but theunit 22 again functions to limit the current flow to a very small valueand so prevents tinkling of the bell. When the dial contacts re-make,the potential is reconnected to the line side of condenser 2D whereuponthe unit 22 functions as already described. 2

' Similar considerations apply in the case of a call originated by partyB, the units 22 and I2 serving to eliminate tinkling of the bells ofeither of, the parties on the line.

' It will'be appreciated that owing to the normal high resistance of theunits I2 and 22 the unbalancing effect of the earth connection made tothe party line during dialling and subsequent conversation isnegligible.

Considering now incoming calls to the party line subscribers, when it isdesired to ring party A an earthed ringing current generator isconnected by the final selector of the automatic switch train to thenegative leg of the party line, whereupon the full '75 volts ringingvoltage is applied across condenser I0, bell II and non-linearresistance I2. The unit I2 being normally of very high resistanceabsorbs practically all the applied voltage, whereupon it breaks down onthe firstpeak and rapidly lowers its resistance. The breakdown conditionpersists until the applied A. C. voltage reahces a low value, whereuponthe unit restores to its normal high resistance and it then subsequentlybreaks down on requirednumber, the dial impulse contacts IMP,

in opening on thefirst break impulse, removethe shunt circuit fromcondenser Ill which now pro ceedsfto charge up over the followingcircuit: battery atthe main exchange, negative line,',con-

denser III, contacts SHI, resistance vYA, induction coil winding I02,resistance YB, induction coil'winding ICI, contacts SHE, and positiveline toearth at the main exchange. A charging path for the condenser isalso completed via bell II andunit I2 to earth, but owing to theshunting effect of the previously mentioned comparatively low resistanceearth circuit via contactsSHI, the voltage developed across unit I2 willbe well below that required: to produce breakdownthereoffendconsequently it maintains its very high resistancewhich preventstinkling of the bell. When jthedial impulsing contacts reclose, the

I condenser Iqis again shunted down and on the nextpopening of thecontacts IMP' the cycle of operations already described is repeated butthe. bel1"I'I] again fails to tinkle owing to the resist anCeofunitIZ. rAs regards the bell'2 I I of party line subscriber B, the condenser .20is normallyfin a discharged condition since it is connected to theearthed positive line and remains so until the A partys receiver islifted when a potential with respect to earth of approximately half theexchange battery voltage appears on the line side of condenser,

due .to the current flow from earth at the ex'-} change, positive line,.A. partys telephone loop,

andnegative line to battery at the exchange.

This potential is oicourse insufficient tojbrea'k down thenon-linearfresistance.unit 22' and the charging current "through thecondenser. 20 will therefore be so small that bell 2| will not tiiikleQwhen the 'A party dials thel first "digit r the se qired number the dialsprings in openin'g re-e move the potential from the line side of condenser,- 20 and if' any charge has accumulated thereon, it. now; tendsto discharge over the p cs-1 is desired to ring the bell of party linesubs'criber B, ringing current is applied at the exchange to thepositive leg of the line, whereupon unit 22 functions to allow bell 2|to operate satisfactorily in the same manner as unit I2 for party A.

The invention is not limited in scope to the earthed bell party linesystem described as it may also be applied with advantage to party linesystems using different ringing frequencies for the various parties on aparty line. In this case differently tuned bells are each connected inseries with a condenser acrossthe party line and the insertion of anon-linear resistance device in series with each condenser and bell willserve to avoid tinkling .due to dialling or extraneous connections andwill also improve the immunity of each'circuit to ringing current offrequencies to which it is not tuned.

The invention also has a sphere of usefulness for ordinary substationsemploying a single telephone instrument per line, particularly where thesignalling bell is permanently. connected across the line byway of acondenser and a separate condenser is employed for anti-side tone'andspark ouench purposes. In this case thefibell condenser may be replacedby asuitable, nonlinear resistance device, since this will perform allthe direct current isolating functions of the condenser and at the sametime prevent anypossible bell tinkling due ;to the causes described, 7}What I claim as new and, desire to secure by Letters Patent is:

1. In a telephone substation, antincoming'line upon which aringingsignal of certain voltage is impressed at times and upon which a .loweiivoltage is impressed at other times; a condenser'andi said callingdevice, a solid body of non-linear resistance material in series withsaid ringer presenting a high resistance to the flow of charging anddischarging current through said ringer thereby to prevent actuation ofsaid ringer when said calling device is operated, the resistance of saidbody being effective to decrease responsive to said certain voltagebeing impressed upon said line, thereby to permit said ringer to respondto said ringing signal.

2. In a telephone system, a party line to which a certain potential isnormally connected at the exchange and upon each side of which a ringingsignal of a higher potential is sometimes impressed at the exchange, twosubscribers stations on said line, one of said stations having acondenser and a ringer connected in series to one side of said line, theother of said stations having a ringer and condenser connected in seriesto the other side of said line, a calling device at one of said stationsoperated at times to transmit impulses over said line to the exchange, acharging circuit and a discharging circuit for the condenser at eachstation completed alternately over the ringer at the same stationwhenever the calling device at said one station is operated, a solidbody of non-linear resistance material in series with each ringerpresenting a high resistance to the fiow of charging and dischargingcurrent through that ringer, thereby to prevent actuation of bothringers when the calling device at said one station is operated, theresistance of each body being effective to decrease responsive to saidcertain potential being impressed upon the side of said line to whichits associated ringer is connected, thereby to permit said associatedringer to respond to said ringing signal when same is impressed upon theside of said line to which that ringer is connected.

3. In a telephone substation, an incoming line upon which a ringingsignal of certain voltage is impressed at times and upon which a lowervoltage is impressed at other times, a transmitter and a receiver, meansincluding a condenser and an induction coil for connecting saidtransmitter and said receiver to said line in an anti-side tone speechcircuit, a solid body of non-linear resistance material, a ringer, aringing circuit connected to said line and including in series saidcondenser, said body and said ringer, said body presenting a highresistance to the flow of current through said ringer whenever saidlower voltage is applied to said line but reducing its resistance to alow value in the presence of said certain voltage, thereby to permitsaid ringer to respond to said ringing signal.

4. In a telephone substation, an incoming line upon which a ringingsignal of certain voltage is applied at times and upon which a lowervoltage is applied at other times, a solid body of nonlinear resistancematerial, and a ringer connected to said line through said resistancematerial, said material presenting a high resistance to the flow ofcurrent through said ringer whenever said lower voltage is applied tosaid line but reducing its resistance to a low value in the presence ofsaid certain voltage, thereby to permit said ringer to respond to saidringing signal.

5. A substation as claimed in claim 4, wherein said non-linearresistance material comprises a mixture of silicon carbide with carbonand/or tungsten which has been agglomerated under pressure andsubsequently baked.

EMANUEL FRYDMAN.

